2007-2008 Nervous System Evolution of the Nervous System 2 Copyright © The McGraw-Hill Companies,...

2007-2008

Nervous System

Evolution of the Nervous System

2

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

nerve net

transversenerves

lateralnervecords

cerebralganglia

brain

eyespotauricle

nerve

ventral nervecord with ganglia

a. Hydra b. Planarian c. Earthworm


3


brain

tentacle

brain

eye

thoracicganglion

giantnervefiber

cerebrumin forebrain

hindbrain

spinalcord

f. Cat e. Squidd. Crab


• Vertebrate Nervous-System Organization– Central nervous system

• Develops from an embryonic dorsal neural tube• Cephalization and bilateral symmetry result in

several paired sensory receptors– Eyes, ears, olfactory structures

– Vertebrate brain is organized into three areas• Hindbrain • Midbrain• Forebrain

4


• Division of Nervous System:– The Central nervous system (CNS)

• Includes the brain and spinal cord– The peripheral nervous system (PNS)

• Consists of all nerves and ganglia that lie outside the CNS

• Two divisions– Somatic nervous system

» Sensory and motor functions that control skeletal muscle

– Autonomic nervous system» Controls smooth muscle, cardiac, muscle, and gland» Divided into sympathetic and parasympathetic divisions

5

Organization of the Human Nervous System

6

radial nerve

median nerve

ulnar nerve

a.

sciatic nerve

tibial nerve

spinal cord

cervical nerves

brain

cranial nerves

common fibularnerve

thoracicnerves

lumbarnerves

sacralnerves


Organization of the Human Nervous System

7

b.

Central NervousSystem brain and

spinal cord

Peripheral NervousSystem

autonomic motorfibers (to cardiac

and smoothmuscle, glands)

sympatheticdivision

parasympatheticdivision

visceral sensoryfibers (internal

organs)

somatic sensoryfibers (skin,

special senses)

somatic motorfibers (to skeletal

muscles)


37.2 Nervous Tissue

• Neurons (nerve cells)

– Cell body contains nucleus and organelles

– Dendrites receive signals from sensory receptors or other neurons

– Axon conducts nerve impulses to another neuron or to other cells

• Covered by myelin sheath

• Any long axon is also called a nerve fiber

8

Nervous Tissue

• Types of Neurons• Motor (efferent) neurons

– Accept nerve impulses from the CNS

– Transmit them to muscles or glands

• Sensory (afferent) neurons – Accept impulses from sensory receptors

– Transmit them to the CNS

• Interneurons– Convey nerve impulses between various parts of the

CNS

9

Neuron Anatomy

10

node of Ranvier

a. Motor neuron (multipolar)

cell body dendrite

axon

muscle

directionof conduction

myelinsheath

axonterminal


a: © M.B. Bunge/Biological Photo Service

11

cell body

b. Sensory neuron (unipolar)

axon

axon

skin

myelin sheath

direction ofconduction

sensoryreceptor


Neuron Anatomy

12c: © Manfred Kage/Peter Arnold, Inc.

c. Interneuron (multipolar)

axon

cell body

dendrite

Neuron AnatomyCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Nervous Tissue

• Transmission of Nerve Impulses– Resting Potential

• The membrane potential (voltage) when the axon is not conducting an impulse

– The inside of a neuron is more negative than the outside, around -70 mV

– Due in part to the activity of the sodium-potassium pump

13

Nervous Tissue

• An action potential is a rapid change in polarity across a portion of an axonal membrane

• An action potential is generated only after a stimulus larger than the threshold

• Gated channel proteins– One channel protein suddenly allows sodium

to enter the cell– Another channel protein allows potassium to

leave the cell

14

Cells: surrounded by charged ions• Cells live in a sea of charged ions

– anions (negative)• more concentrated within the cell• Cl-, charged amino acids (aa-)

– cations (positive)• more concentrated in the extracellular fluid• Na+

Na+ Na+ Na+ Na+ Na+ Na+ Na+Na+ Na+ K+ Na+ Na+

Cl-

K+ Cl- Cl- Cl-K+

aa-K+ Cl- Cl-

aa- aa-aa-

aa- aa-K+

K+channel leaks K+

channel leaks K+ +

–

Cells have voltage!• Opposite charges on opposite sides of cell

membrane– membrane is polarized

• negative inside; positive outside• charge gradient• stored energy (like a battery)

+ + + + + + + ++ + + + + + +

+ + + + + + + ++ + + + + + +

– – – – – – – ––– – – – –

– – – – – – – ––– – – – –

Measuring cell voltage

unstimulated neuron = resting potential of -70mV

How does a nerve impulse travel?• Stimulus: nerve is stimulated

– reaches threshold potential • open Na+ channels in cell membrane• Na+ ions diffuse into cell

– charges reverse at that point on neuron• positive inside; negative outside • cell becomes depolarized

– + + + + + + ++ + + + + + +

– + + + + + + ++ + + + + + +

+ – – – – – – –– – – – – – –

+ – – – – – – –– – – – – – –Na+

The 1stdomino goesdown!

Gate

+ –

+

+

channel closed

channel open

How does a nerve impulse travel?• Wave: nerve impulse travels down neuron

– change in charge opens next Na+ gates down the line • “voltage-gated” channels

– Na+ ions continue to diffuse into cell– “wave” moves down neuron = action potential

– – + + + + + +– + + + + + +

– – + + + + + +– + + + + + +

+ + – – – – – –+ – – – – – –

+ + – – – – – –+ – – – – – –Na+

wave

The restof thedominoes fall!

How does a nerve impulse travel?• Re-set: 2nd wave travels down neuron

– K+ channels open• K+ channels open up more slowly than Na+ channels

– K+ ions diffuse out of cell– charges reverse back at that point

• negative inside; positive outside

+ – – + + + + +– – + + + + +

+ – – + + + + +– – + + + + +

– + + – – – – –+ + – – – – –

– + + – – – – –+ + – – – – –Na+

K+

wave

Setdominoesback upquickly!

How does a nerve impulse travel?• Combined waves travel down neuron

– wave of opening ion channels moves down neuron– signal moves in one direction

• flow of K+ out of cell stops activation of Na+ channels in wrong direction

+ + – – + + + ++ – – + + + +

+ + – – + + + ++ – – + + + +

– – + + – – – –– + + – – – –

– – + + – – – –– + + – – – –Na+

wave

K+Readyfornext time!

How does a nerve impulse travel?• Action potential propagates

– wave = nerve impulse, or action potential– brain finger tips in milliseconds!

+ + + + – – + ++ + + – – + +

+ + + + – – + ++ + + – – + +

– – – – + + – –– – – + + – –

– – – – + + – –– – – + + – –Na+

K+

wave

In theblink ofan eye!

Voltage-gated channels• Ion channels open & close in response to changes in

charge across membrane

– Na+ channels open quickly in response to depolarization & close slowly

– K+ channels open slowly in response to depolarization & close slowly

+ + + + + – + ++ + + + – – +

+ + + + + – + ++ + + + – – +

– – – – – + – –– – – – + + –

– – – – – + – –– – – – + + –Na+

K+

wave

Structure& function!

How does the nerve re-set itself?• After firing a neuron has to re-set itself

– Na+ needs to move back out– K+ needs to move back in– both are moving against concentration gradients

• need a pump!!

+ + + + + – – ++ + + + + – –

+ + + + + – – ++ + + + + – –

– – – – – + + –– – – – – + +

– – – – – + + –– – – – – + +Na+

Na+Na+

Na+ Na+Na+

K+K+K+K+ Na+ Na+

Na+Na+Na+

Na+Na+

Na+Na+

Na+

Na+

K+K+K+K+

K+K+

K+ K+

wave

K+

Na+

A lot ofwork todo here!

How does the nerve re-set itself?• Sodium-Potassium pump

– active transport protein in membrane• requires ATP

– 3 Na+ pumped out– 2 K+ pumped in– re-sets charge

across membrane

ATP

That’s a lot of ATP !Feed me somesugar quick!

Neuron is ready to fire againNa+ Na+ Na+ Na+ Na+ Na+ Na+

Na+ Na+ Na+ Na+ Na+ Na+

Na+ Na+ Na+ Na+ Na+ Na+ Na+

Na+ Na+ Na+ Na+ Na+ Na+

K+

K+ K+ K+ K+

K+

aa-K+ K+ K+

aa- aa-aa-

aa- aa-

+ + + + + + + ++ + + + + + +

+ + + + + + + ++ + + + + + +

– – – – – – – –– – – – – – –

– – – – – – – –– – – – – – –

resting potential

1. Resting potential2. Stimulus reaches threshold

potential3. Depolarization

Na+ channels open; K+ channels closed

4. Na+ channels close; K+ channels open

5. Repolarizationreset charge gradient

6. UndershootK+ channels close slowly

Action potential graph

–70 mV

–60 mV

–80 mV

–50 mV

–40 mV

–30 mV

–20 mV

–10 mV

0 mV

10 mV DepolarizationNa+ flows in

20 mV

30 mV

40 mV

RepolarizationK+ flows out

ThresholdHyperpolarization(undershoot)

Resting potential Resting1

2

3

4

5

6

Mem

bra

ne

po

ten

tial

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Resting and Action Potential of the Axonal Membrane

d. An action potential can be visualized if voltage changes are graphed over time.

restingpotential

threshold

rep

ola

rizatio

n

K+ movesto outsideaxon

Na+ movesto insideaxon

actionpotential

Vo

ltag

e (m

V)

dep

ola

riza

tio

n

0

0 1 2 3 4 5 6

Time (milliseconds)

+60

–60

–40

–20

+20

+40


Myelin sheath

signaldirection

Axon coated with Schwann cells insulates axon speeds signal

signal hops from node to node saltatory conduction

150 m/sec vs. 5 m/sec(330 mph vs. 11 mph)

myelin sheath

myelin

axon

Na+

Na+

++ + + + –

–

action potential

saltatoryconduction

Multiple Sclerosis immune system (T cells)

attack myelin sheath loss of signal

Multiple Sclerosis immune system (T cells)

attack myelin sheath loss of signal

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Nervous Tissue

• Propagation of Action Potentials– In nonmyelinated axons, the action potential travels

down an axon one small section at a time– In myelinated fibers, an action potential at one node

causes an action potential at the next node• Saltatory (jumping) Conduction

– Conduction of a nerve impulse is an all-or-nothing event

• Intensity of signal is determined by how many impulses are generated within a given time span

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32

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Animation

Animation


Nervous Tissue

• Transmission Across a Synapse

– A synapse is a region where neurons nearly touch

– Small gap between neurons is the synaptic cleft

– Transmission across a synapse is carried out by neurotransmitters

• Sudden rise in calcium in the axon terminal of one neuron

• Calcium stimulates synaptic vesicles to merge with the presynaptic membrane

• Neurotransmitter molecules are released into the synaptic cleft

35

Synapse Structure and Function

36

path of action potential

synaptic cleft

receptor

neurotransmitter

Na+

Ca2+

1. After an action potential arrives at an axon terminal, Ca2+ enters, and synaptic vesicles fuse with the presynaptic membrane.

cell body ofpostsynapticneuron

axonterminal

synaptic vesiclesenclose neuro-transmitter

2. Neuro- transmitter molecules are released and bind to receptors on the postsynaptic membrane.

presynapticmembrane

postsynapticmembrane

3. When an excitatory neurotransmitter binds to a receptor, Na+ diffuses into the postsynaptic neuron, and an action potential begins.

neuro-transmitter

postsynapticneuron


Animation


Synapse

Impulse has to jump the synapse!– junction between neurons– has to jump quickly from one cell to next

What happens at the end of the axon?

How does the wavejump the gap?

axon terminal

synaptic vesicles

muscle cell (fiber)

neurotransmitteracetylcholine (ACh)receptor protein

Ca++

synapse

action potential

Chemical synapse Events at synapse

action potential depolarizes membrane

opens Ca++ channels neurotransmitter vesicles

fuse with membrane release neurotransmitter

to synapse diffusion neurotransmitter binds

with protein receptor ion-gated channels open

neurotransmitter degraded or reabsorbed

We switched…from an electrical signalto a chemical signal

Nerve impulse in next neuron • Post-synaptic neuron

– triggers nerve impulse in next nerve cell• chemical signal opens ion-gated channels • Na+ diffuses into cell• K+ diffuses out of cell

– switch back to voltage-gated channel

– + + + + + + ++ + + + + + +

– + + + + + + ++ + + + + + +

+ – – – – – – –– – – – – – –

+ – – – – – – –– – – – – – –Na+

K+

K+K+

Na+ Na+

Na+

ion channel

binding site ACh

Here wego again!

Nervous Tissue

• Synaptic Integration– A single neuron is on the receiving end of

• Many excitatory signals, and• Many inhibitory signals

– Integration• The summing of excitatory and inhibitory signals

41

Synaptic Integration

42(a): Courtesy Dr. E.R. Lewis, University of California Berkeley

a.

b.

threshold

restingpotential

axon terminalscell body of the neuron

excitatory signal

integrationinhibitory signal

Time (milliseconds)

+20

–20

–40

–70

–80

0


37.3 The Central Nervous System

• The Central Nervous System– Consists of the brain and spinal cord– Three specific functions:

• Receives sensory input• Performs integration• Generates motor output

43

The Central Nervous System

• The Central Nervous System– Consists of the brain and spinal cord– Spinal cord and brain are wrapped in three

protective membranes called meninges• Spaces between meninges are filled with

cerebrospinal fluid• Fluid is continuous with that of central canal of

spinal cord and the ventricles of the brain

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• The Spinal Cord– Two main functions

• Center for many reflex actions• Means of communication between the brain and

spinal nerves

– Composed of grey matter and white matter• Cell bodies and short unmyelinated fibers give the

gray mater its color• Myelinated long fibers of interneurons running in

tracts give white mater its color

45


• The Brain– The cerebrum is the largest portion of the

brain in humans• Communicates with, and coordinates the activities

of, the other parts of the brain• Longitudinal fissure divides the cerebrum into left

and right cerebral hemispheres

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The Human Brain

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skull

meninges

third ventricle

pituitary gland

pineal gland

fourth ventricle

spinal cord

Diencephalon

Cerebellum

hypothalamus

midbrain

pons

Brain stem

b. Cerebral hemispheresa. Parts of brain

Cerebrum(telencephalon)

corpuscallosum

medullaoblongata

opening to lateralventricle

thalamus(surrounds thethird ventricle)


The Lobes of a Cerebral Hemisphere

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Frontal lobe

Occipital lobe

Parietal lobe

premotor area

trunk

armhand

face

tongue

leg

primary motor area

motor speech(Broca’s) area

prefrontalarea

central sulcus

primary somatosensory area

somatosensoryassociation areaprimary taste area

general interpretation area

primaryvisual area

visualassociationarea

lateral sulcus

Temporal lobe

auditory association area

primary auditory area

sensory speech (Wernicke’s) area



• Cerebral Cortex• A thin but highly convoluted outer layer of

gray matter• Covers the cerebral hemispheres• Contains motor areas and sensory areas as

well as association areas– Primary motor area is in the frontal lobe, just

ventral to the central sulcus– Primary somatosensory area is in the parietal lobe,

just dorsal to the central sulcus

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• Other Parts of the Brain• Diencephalon

• A region encircling the third ventricle

• Includes three structures

– Hypothalamus

• Forms the floor of the third ventricle

• Integrating center that maintains homeostasis

• Controls the pituitary gland

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• Other Parts of the Brain

• Diencephalon (continued)• Thalamus

• Consists of two masses of gray matter located in the sides and roof of the third ventricle

• Receives all sensory input except smell

• Integrates sensory information and sends it to the cerebrum

• Pineal gland• Secretes melatonin

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• Other Parts of the Brain• Cerebellum

• Separated from the brain stem by the fourth ventricle

• Largest portion of the brainstem

• Receives sensory input from the eyes, ears, joints, and muscles

• Sends motor impulses out the brain stem to the skeletal muscles

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• Other Parts of the Brain• Brain Stem

• Contains the midbrain, pons, and the medulla oblongata

• Midbrain • Acts as a relay station for tracts passing between the

cerebrum and the spinal cord or cerebellum• Pons

• Contains axons that form a bridge between the cerebellum and the rest of the central nervous system

• Medulla Oblongata• Contains reflex centers for vomiting, coughing,

sneezing, hiccupping, and swallowing

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37.4 The Peripheral Nervous System

• Somatic system– Includes cranial nerves and spinal nerves

• Gather information from sensors and conduct decisions to effectors• Controls the skeletal muscles

– Voluntary

• Autonomic system– Controls the smooth muscles, cardiac muscles, and glands

– Innervates all internal organs

– Usually involuntary

– Divided into two divisions• Sympathetic division

• Parasympathetic division

– Utilizes two neurons and one ganglion for each impulse

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Cranial and Spinal Nerves

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dorsal root

ventral root

vertebra

spinal cord

white matter

central canal

gray matter

frontal lobe

olfactory bulb

olfactory tract

optic chiasma

temporal lobe

optic nerve

cerebellum

medulla

gray matter

white matter

vertebra

b.

c.

a.

dorsal rootganglion

spinalnerve


c: © Karl E. Deckart/Phototake

The Peripheral Nervous System

• Reflex Arc• Sensory receptors generate a nerve impulse that moves along

sensory axons through a dorsal root ganglion toward the spinal cord

• Sensory neurons pass signals on to many interneurons in the gray matter of the spinal cord

• Nerve pulses travel along motor axons to an effector, which brings about a response to the stimulus

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A Reflex Arc Showing thePath of a Spinal Reflex

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white matter

ventral root

axon of motor neuron

axon of sensory neuron

pin

dorsal root ganglion

interneuron

dendrites

dendrites Dorsal gray matter

central canal

ventral horn

sensoryreceptor(in skin)

cell body ofsensory neuron

cell body ofmotor neuron

effector(muscle)

Ventral

dorsalhorn


The Peripheral Nervous System

• Sympathetic division– Especially important during fight or flight

responses– Accelerates heartbeat and dilates bronchi

• Parasympathetic division– Promotes all internal responses associated

with a relaxed state– Promotes digestion and retards heartbeat

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Autonomic System Structure and Function

59

inhibits tears stimulates tears

constricts pupils

gangliondilatespupils

Sympathetic Division

inhibits salivation

stimulatessalivation

cranialnerves

Parasympathetic Division

slows heart

speedsheart

dilates airpassages

cervicalnerves

thoracicnerves

lumbarnerves

constrictsbronchioles

stimulates liver torelease glucose

stimulatesadrenalsecretion

vagus nerve

inhibits activityof kidneys,stomach, andpancreas

increasesintestinalactivity

decreasesintestinal activity

ganglioninhibitsurination

stimulatesurination

sacralnerves

stimulates gallbladderto release bile

increases activityof stomach andpancreas

sympathetic ganglia

causesorgasmiccontractions causes

erectionof genitals

Acetylcholine is neurotransmitter.Norepinephrine is neurotransmitter.


Comparison of Somatic Motor and Autonomic Motor Pathways

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2007-2008 Nervous System Evolution of the Nervous System 2 Copyright © The McGraw-Hill Companies,...

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Transcript of 2007-2008 Nervous System Evolution of the Nervous System 2 Copyright © The McGraw-Hill Companies,...